The invention concerns a shielded module support structure for printed circuit boards which can be inserted on guide rails and having electrical or electronic components. The module support structure comprises two parallel metallic side walls and at least four parallel metallic module rails connecting the side walls and bearing the guide rails. The printed circuit boards are assembled into plug-in modules, each having a front plate, which can be inserted and removed, wherein the front module rails each have a groove opening towards the front of the module support structure for accepting a threaded-hole-strip and each also having at least one seating surface for the front plates of the plug-in modules.
Electronic circuits, in particular digital ones, have increasingly lower switching thresholds and are sensitive to interfering ambient radio frequency fields. For this reason, module support structures having printed circuit boards bearing this kind of sensitive circuitry must be shielded at all sides. Towards this end, one uses radio frequency sealed side walls and upper and lower sheet metal as well as rear covers. Shielding with respect to interfering radio frequency fields in the region of the front plate is, however, more problematic since individual printed circuit boards should be removable at any time. Particularly when the module support structure does not have a common front plate extending over the entire width of the module support structure, rather accepts plug-in modules having the inserted printed circuit boards mounted to individual front plates, the gaps between the front plates and the module support structure cause special problems, particularly at the module rails.
Shielding of the vertical gaps between the front plates and between the front plates and the side walls is usually effected using resilient elements as e.g. described in the patent publication DE 41 10 800 C1, due to the applicant. Herein, resilient metallic elements are placed on the legs of a U-shaped front plate and press against the neighboring front plate or against the side wall of the module support structure when the plug-in module is inserted to thereby effect the required low resistance area contact.
The most differing of approaches have been taken for the horizontal contacting of the front plates to the module rails. Known in the art from e.g. DE 41 26 576 A1 is a resilient element which is inserted into the groove, opening towards the front, of a module rail and seats in a widened portion of the groove disposed in front of the threaded-hole-strip. The resilient element comprises an angled strip which protrudes in a forward direction beyond the groove in the module rail and is displaced towards the threaded-hole-strip by the front plate in opposition to an elastic resilient force when the plug-in module is inserted.
A contact element is also known in the art from DE 296 02 426 U1 which seats in the groove, opening towards the front, of a module rail and is disposed in front of the threaded-hole-strip. In this prior art, the contact with the front plate of the plug-in module is effected via periodically disposed contact spikes projecting in a forward direction beyond the module rail.
The conventional contact elements have the common feature that they seat in the grooves of the module rails between the front plate and the threaded-hole-strip. The screws for securing the plug-in modules must therefore be screwed-in through the contact elements into the threaded-hole-strip. The contact elements must therefore have holes matched with the pattern of the threaded-hole-strips. In addition, these holes must align with the threaded holes. This requires a significant amount of additional effort when assembling a module support structure of this kind.
In addition to the introduction of contact elements in the groove of a module rail, it is also in principle possible to provide the seating surfaces of the module rails with contact elements. One must thereby take into account the fact that the seating surfaces of the module rails serve as contact surfaces for plug-in module removal means. With plug-in modules having triple-row equipped, 96-terminal plugs, the removal force exercised by a plug-in module removal means of this kind on the seating surface of the module rail can assume values of up to 100 N. The developments in electronic printed circuit boards will thereby lead to an additional increase in the necessary removal forces in the near future.
Conventional shielding elements disposed in the region of the seating surface of the module rails are therefore glued to the inside of the front plate and the removal means region is cut-away. This approach requires a very precise gluing of the shielding elements, since they must precisely seat on the seating surface of the module rails when the plug-in module is inserted.